A vinyl chloride polymer is a polymer including 50% or more of vinyl chloride, is cheap, allows easy control of hardness, is applicable in most processing equipments and has diverse application fields. In addition, the vinyl chloride polymer may provide a molded product having good physical and chemical properties such as mechanical strength, climate resistance and chemical resistance, and is widely used in diverse fields.
However, the vinyl chloride polymer may induce dehydrochlorination due to the defects of chemical structures, generated during a polymerization reaction by heat or ultraviolet rays applied during processing and may induce the discoloration of a resin or the deterioration of physical properties.
Particularly, in a vinyl chloride polymer, chemical defects generated during a polymerization reaction, i.e., the defects of the chemical structures of allyl chloride and tertiary chloride are present. Due to the defects of the chemical structure, the binding energy of carbon with chlorine in the vinyl chloride polymer may be much lower than that of carbon with chlorine in a normal molecular structure, and the bonding between carbon and chlorine may be easily broken due to external radical transfer during processing the vinyl chloride polymer. Separated hydrogen chloride from a polymer chain accelerates other side reactions through an auto-catalyst reaction to produce another hydrogen chloride continuously. In addition, a double bond may be formed at the site from where the hydrogen chloride has been removed, and a number of double bonds may be overlapped to generate the discoloration of a resin and the deterioration of physical properties. That is, in a vinyl chloride polymer or a molded product processed therefrom, a dehydrochlorination reaction may be generated due to heat or ultraviolet rays, thereby generating the discoloration defects of a vinyl chloride polymer or the deterioration or change of physical properties.
In order to improve the limitations of the vinyl chloride polymer, an organometallic compound containing a metal such as Ba, Zn, Ca and Pb has been mixed with the vinyl chloride polymer to restrain the generation of radicals or ions during thermal decomposition of the vinyl chloride polymer and to control the thermal decomposition rate of a resin. Recently, a method of using a thermal stabilizer of diverse types such as a metallic material or an organic compound has been introduced. However, the use thereof is limited due to environmental problems caused by the use of a heavy metal stabilizer and high price.
A method of blending a polymer having good heat resistance with a vinyl chloride polymer has been suggested to complement weak physical properties however is not easily used due to low miscibility with the vinyl chloride polymer and difficulty in processing.
With such a background, the inventors of the present disclosure studied on a method of improving the thermal stability of a vinyl chloride polymer by effectively restraining a dehydrochlorination reaction (a dehydrochloric acid reaction), prepared a vinyl chloride polymer by adding at least one modifier of oxycarboxylates, inorganic phosphates or ethylenediamine tetraacetates to a vinyl chloride monomer and bulk polymerizing, measured the generation degree of scale, the amount of dehydrochloric acid and thermal stability (Blackening index), found that the generation degree of the scale is extremely small, the amount of dehydrochloric acid is decreased, and the thermal stability (Blackening index) is improved, and achieved the present invention.